1. Field of the Invention
The invention relates to a process for producing low density polyethylenic foam.
2. Background Art
Low-density thermoplastic foams, particularly those having a density of less than about 150 kg/m.sup.3, generally are produced with physical blowing agents. Physical blowing agents are those chemical compounds which can be incorporated into the thermoplastic melt while within the extruders at high pressures, typically at 10 to 20 MPa, and which can be contained by the polymer structure when the cooled thermoplastic/blowing agent mixture is rapidly reduced to ambient atmospheric pressure. Polyethylenic foams, in particular uncrosslinked polyethylene foam, were manufactured for many years with halogenated hydrocarbons, primarily chlorofluorocarbons (CFC's), as physical blowing agents.
During the 1980's, the worldwide scientific community presented sufficient evidence linking CFC's with atmospheric ozone depletion and sought for governments to regulate CFC's. Ozone levels in the stratosphere have been shown to be significant in protecting life on the planet from the damaging effects of ultraviolet radiation from the sun. Additionally, hydrochlorofluorocarbons (HCFC's), another class of chemical compounds, were also included in the resultant adopted regulations, but on a more lengthy schedule. The scientific community also showed that it was the higher atomic weight halogen compounds other than fluorine, which were the culprits.
As a result of the regulations, many low-density foam manufacturers sought to find materials other than halogenated compounds which could function as physical blowing agents. Many different approaches were taken, but generally these approaches involved the use either of hydrocarbons containing from two to five carbons or of blends thereof. Such short-chained hydrocarbons generally function as physical blowing agents to produce foams with satisfactory to physical properties, but they do have the adverse effect of flammability. U.S. Pat. No. 4,217,319 (Komori) discloses a process for producing polyethylene foams with various volatile organic compounds as the physical blowing agent. U.S. Patent Nos. 5,290,822 and 5,225,451 (Rogers et al.) disclose processes for producing ultra-low density foams using polymer blends. U.S. Pat. No. 5,059,376 (Pontiff) discloses a process to remove residual hydrocarbons, the purpose of which is to produce a non-flammable polyethylene foam.
U.S. Pat. No. 4,323,528 (Collins) discloses a method and an apparatus for manufacturing a large size, low density foam sheet of thickness in excess of 25 mm without restriction of the physical blowing agent. The intermittent process disclosed in the Collins patent provides an alternative embodiment for the current disclosure.
Patents for several processes involving the use of atmospheric gases to produce polyethylene foam have been issued in the 1990's. For example, U.S. Pat. No. 5,416,129 (Chaudhary et al.) discloses a process for preparing uncrosslinked ethylenic polymer foam using a polymer of a defined melt tension with an inorganic halogen-free blowing agent, such as, argon, carbon dioxide, or blends thereof. This disclosure indicates that argon and carbon dioxide in blends can function as a physical blowing agent with ethylenic materials having a specific melt tension. However, the associated reported data therein also suggest difficulty in maintaining satisfactory long term foam quality.
Patented processes for the purification of hydrocarbon effluent gases have utilized the enhanced solubility of acid gases in certain polyethers such as in tetraethylene glycol dimethyl ether. U.S. Pat. No. 4,421,535 (Mehra) and U.S. Pat. No. 4,695,672 (Bunting) describe processes which utilize physical solvents such as tetraethylene glycol dimethyl ether to remove acid gases such as carbon dioxide from hydrocarbon gas streams. These disclosures demonstrate the high affinity of the ether chemical bond for certain gases.